Task: expand context abstractions for scope and factory contracts

[rian-be]

Summary

Expand ModularityKit.Context.Abstractions with the missing core contracts needed for a more complete and predictable context package.

Goal

Make the abstraction layer match the actual runtime capabilities of the library and provide stable contracts for context creation, scoping, and future integration points.

Problem

ModularityKit.Context.Abstractions is currently too small for a package that already supports ambient context access, scoped execution, read-only projection, and execution tracing.

The current abstractions cover the basics, but they do not yet give consumers a clean contract for:

• manual scope control
• centralized context creation
• future diagnostic integration
• explicit lifecycle modeling

That makes the package usable, but under-specified.

Scope

• Add a first-class IContextScope<TContext> abstraction
• Add a IContextFactory<TContext> abstraction for centralized context creation
• Add lifecycle-oriented contracts only where they are useful in the core package
• Keep the abstraction layer aligned with the current runtime and model layers
• Avoid duplicating model types as interfaces when a record or enum is a better fit
• Update public examples and docs so the new abstractions are discoverable

Design Expectations

• Abstractions should describe stable concepts, not implementation details.
• The surface should stay small enough to remain approachable.
• New contracts should improve composition without forcing extra layers on simple users.
• Models should remain models when they do not need interface-based polymorphism.

Acceptance Criteria

• The abstraction layer includes a clear scope contract.
• The abstraction layer includes a factory contract for context creation.
• The new abstractions map cleanly to the existing runtime APIs.
• The package remains small and predictable instead of becoming over-abstracted.

Non-Goals

• No governance layer.
• No policy engine.
• No event bus.
• No persistence backend.
• No ASP.NET-specific integration.

Notes

This issue is about making the abstraction layer match the library’s actual shape, not about turning the package into a large framework.